This invention relates to the photoelectrophoretic imaging process and more particularly to an improved process wherein the imaging suspension is uniformly charged.
A detailed description of the photoelectrophoretic imaging process and materials and apparatus therefor appears in U.S. Pat. Nos. 3,383,993; 3,384,488; 3,384,565 and 3,384,566. The disclosures of the aforementioned patents are hereby incorporated by reference. Briefly, the photoelectrophoretic imaging process, as described in the aforementioned incorporated patents, is a method wherein a liquid suspension of electrically photosensitive particles is placed between a pair of electrodes. The particles acquire a charge when an electrical field is placed between the electrodes which charge is modified by exposure of the particles to light thus causing a light controlled deposition of the particles on one boundary of the suspension or the other. Particle movement is caused by the force exerted on the charged particles by the electric field. The light absorbed by a particle enables it to undergo a change in polarity which then determines its position in the field. One of the electrodes in the process is termed a conductive electrode which is generally a transparent conductive material and is the electrode upon which the pigments desirably rest at the time they are exposed to appropriate electromagnetic radiation. While not subscribing to any particular theory, the aforementioned patents propose that the pigments when exposed to actinic electromagnetic radiation while resting upon the conductive electrode, acquire a charge from the electrode. Upon acquisition of such charge the particle moves toward the opposite electrode. The opposite electrode is generally covered with an electrically insulating material such that when a pigment particle contacts the electrode under influence of the field it will not give up any charge and will remain against the blocking electrode. Upon separation of the electrodes there is generally provided an optically positive image on one of the electrodes and a negative image residing on the other electrode either monochromatic or polychromatic depending upon the optical input and the colors of the pigments in the imaging suspension.
As previously mentioned, the pigments in the imaging suspension have an initial charge and also can acquire an additional electrical charge upon being subjected to an electrical field between the electrodes. One of the problems encountered in the above-mentioned process relates to the polarity of the charge acquired by the pigments of any one color. For example, while about half of certain magenta pigments particles may exhibit a negative charge in the field between the electrodes while in the dark, the other half of the pigment particles will acquire the opposite charge and thus migrate immediately, in the dark, to the blocking electrode. Thus because only some of the pigment resides on the conductive electrode, the density of the resultant image on the conductive electrode is reduced by the amount of pigment deposited on the blocking electrode. Another disadvantage of this phenomenon is the unwanted deposition on the blocking electrode of such pigments in background areas thus degrading image quality of both images produced by the process.
The problem of the non-uniform charging of the pigments in the imaging suspension of the photoelectrophoretic imaging method is well known and, in fact, has been employed advantageously in the prior art. For example, a photoelectrophoretic imaging system taking advantage of the diversity of the dark charge of the pigments is disclosed in U.S. Pat. No. 3,535,221, to Tulagin. In accordance with the system disclosed therein the image sense, optical positive or optical negative, is controlled such that one may produce a positive image or a negative image on either of the blocking electrode or the conductive electrode. The method of selectively producing positive or negative copies on either electrode is achieved by providing an imaging suspension with pigment particles having a sensitivity to a first range of wavelengths and providing on the blocking layer surface a photosensitive material sensitive to a second range of wavelengths. In accordance with the disclosure of that patent an optically positive image is formed on the blocking electrode by exposing the suspension and blocking layer to light in wavelength to which only the coating on the blocking layer is sensitive. If one desires to produce an optically positive image on the conductive electrode one exposes the imaging suspension to electromagnetic radiation to which the particles of the imaging suspension are sensitive but to which the material on the blocking layer is insensitive. The positive image is formed on the blocking layer because of the diversity of charge acquired by the particles of the imaging suspension in the dark. Some of the imaging pigments are attracted to the blocking electrode to form a coating of imaging pigment particles. When the material on the blocking layer is exposed to actinic radiation the pigment particles of the suspension are repelled from the blocking layer in the exposed areas. According to the patent, the coating on the blocking layer reflects back any pigment attracted to it when the coating on the blocking layer is struck with light to which its coating is sensitive. Thus the light exposed areas will contain no pigment while there will reside on the blocking electrode in non-light struck areas a coating of imaging pigment which has taken an opposite charge to those coating the conductive electrode. When the imaging suspension is exposed with light to which it is sensitive, but to which the coating on the blocking layer is insensitive, the imaging pigments coating the conductive layer are caused to migrate to the blocking electrode in the exposed areas. There is thus produced a positive image configuration of the imaging particles on the conductive electrode.
An even more severe problem than the non-uniform charging described above exists with polychromatic images produced by the aforementioned process because the loss of varying amounts of pigments of the different colors in the suspension destroys the color balance intended to produce the desired final result.
We have discovered that certain materials, many of them being pigments previously known to be useful in the imaging suspension of the photoelectrophoretic imaging process, can be placed in a Dark Charge Injection Series. Materials in this series have the ability to inject charge into the pigments of the imaging suspension while they are both under an electrical field in the dark. Generally, those materials higher in the series have the ability to inject charges into those materials lower in the series. Typically, the material employed to inject charges into the pigments of the imaging suspension are termed "dark charge injecting materials" and are placed on the blocking layer of the photoelectrophoretic imaging system for use in that purpose.